Mainlobe doppler clutter return compensator for moving platform radar



1967 w. H. MULLINS ETAL 3,346,859

MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR Filed June 21, 1965 19 Sheets-Sheet l Awgwraa M40444 ,4 Adz/41w;

Arron 5% w. H. MULLINS ETAL 3,346,859 MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR Oct. 10, 1967 FOR MOVING PLATFORM RADAR l9 Sheets-Sheet 2 Filed June 21, 1965 W. H. MULLINS ETAL MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR Oct. 10, 1967 FOR MOVING PLATFORM RADAR l9 Sheets-Sheet 5 Filed June 21, 1965 Oct. 10, 1967 w. H. MULLINS ETAL MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR 19 Sheets-Sheet 4 Filed June 21, 1965 10, 1967 w. H. MULLINS ETAL 3,

MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR Filed June 21, 1965 19 Sheets-Sheet 5 w. H. MULLINS ETAL MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR l9 Sheets-Sheet 6 Oct. 10, 1967 Filed June 21, 1965 W. H. MULLINS ETAL MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR Oct. 10, 1967 FOR MOVING PLATFORM RADAR l9 Sheets-Sheet 7 Filed June 21, 1965 N NN Oct. 10, 1967 w. H. MULLINS ETAL 3,346,859

MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR Oct. 10, 1967 w. H. MULLINS ETAL MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR l9 Sheets-Sheet 9 Filed June 21, 1965 w. H. MULLINS ETAL 3,346,859 7 MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR Oct. 10, 1967 FOR MOVING PLATFORM RADAR 19 SheetsSheet 10 Filed June 21, 1965 R hQux Qmm smm w. H. MULLINS ETAL 3,346,859 MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR Filed June 21, 1965 19 Sheets-Sheet 11 Get. 10, 1967- s/Na 1967 w H. MULLINS ETAL 3,346,859

MAINLOBE DOP PLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR 4 Filed June 21, 1965 19 SheetsSheet 12 Oct. 10, 1 w. H. MULLINS ETAL MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR l9 Sheets-Sheet 13 Filed June 21, 1965 10,1967 w. H. MULLINS ETAL MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR Filed June 21, 1965 19 Sheets-Sheet 14 QQM Oct. 10, 1967 w. H. MULLINS ETAL 3,

MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR Oct. 10, 1967 w. H. MULLINS ETAL 3,345,859

MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR Filed June 21, 1965 19 SheetsS heet 16 w. H. MULLINS ETAL 3,346,859 MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR Oct. 10, 1967 FOR MOVING PLATFORM RADAR 19 Sheets-Sheet 17 Filed June 21, 1965 QN 6mm QMQQ NN Q NN l9 Sheets-Sheet 18 H. MULLINS ETAL W. MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR mm\ W \N\ Oct. 10, 1967 Filed June 21, 1965 Oct. 10, 1967 w. H. MULLINS ETAL 3,

MAINLOBE DOPPLER CLUTTER RETURN COMPENSATOR FOR MOVING PLATFORM RADAR MAINLOBE DOPPLER CLUTTER RETURN COM- ware Filed June 21, 1965, Ser. No. 465,791 6 Claims. (Cl. 3437.7)

This invention relates to moving platform radar systems and particularly to a residual mainlobe Doppler clutter return compensator to provide system immunity from clutter return fluctuations and obscuring effects.

When a coherent radar system is mounted on a moving platform, and the platform is moving at some velocity toward stationary reflectors the phase of the coherent echo return advances 180 degrees for each A wavelength of motion toward the object. Such motion produces the well-known Doppler frequency increase. If the motion is at high speed, then many wavelengths are traversed over a short time. Search radars, while scanning terrain, contain a mainlobe beam which strikes the terrain at varying angles with respect to the ground track or line of sight. Because of such varying angle and terrain configuration, and motion of the platform, a changing Doppler shift from mainlobe return signals require some compensation to make the received return signal from fixed objects to appear stationary relative to the moving platform receiver itself. Unless compensation is performed for the motion of the platform, the presence of a moving target near the terrain scanned by the mainlobe would be obscured by the mainlobe clutter Doppler return on the radar visual display. Compensation for mainlobe clutter Doppler return due to radar platform motions hereinafter shall also be referred to as velocity compensation.

. Prior art methods for velocity compensation of received coherent clutter returns are of two general types, both of which are phase-dependent requiring high tolerance components and limited to a closed-loop operation. The term closed-loop refers to a control system with one or more feedback control loops, in which functions of certain controlled signals are combined with functions of the command signals tending to maintain some prescribed relationship between the command signals and the controlled signals. One of these general types is a fastacting clutter-locked compensator which phase-locks a referenced oscillator to the clutter immediately preceding the target signal. When such velocity compensators operate over land, an extraneous signal from slidelobe ground clutter is also detected which is not representative of the required error signal to cause a loss of effective velocity compensation. This loss of compensation is due to the shift of the mainlobe and the clutter reference toward the Doppler shift of the sidelobe. Therefore, the phase measurements will not be synchronized properly. A second general type of velocity compensator is referred to as a time-averaged, clutter-coherent, phase-locked .velocity compensator. Such a velocity compensator incorporates a network for measuring the average phase of the mainlobe ground return in a given sweep and range gates over the range where clutter is expected. By averaging the detected phase in a given sweep over a relatively long time, the effects of momentary fluctuations in clutter are minimized. But, phase-locking may be transferred from the range-gated mainlobe beam clutter to the sidelobe clutter when operating over built-up heterogeneous terrain, obscuring moving targets in the display with stationary clutter ground returns. Both of these two types of prior art velocity compensators work effectively over homogeneous terrain where a constant reference clutter return is available. However, when either of the prior art United States Patent velocity compensators are to operate over heterogeneous terrain, thus the absence of a constant reference clutter,

effective velocity compensation for clutter during scanning is ineffective.

Thus the main problems encountered with prior art is the loss of effective velocity compensation for clutter during scanning of heterogeneous terrain.

A velocity compensator operable in a moving platform coherent radar which has open-loop operation, frequencydependent rather than phase-dependent, requires lowtolerance components, and is reliable and effective in radar scanning over both homogeneous and heterogeneous terrain is highly desirable. The term open-loop operation as used in this specification refers to a control system in which the controlled quantity is permitted to vary freely with the inherent characteristics of the control system.

It is therefore an object of this invention to provide a velocity compensator in a moving. platform radar, having a coherent receiver, that reliably operates to immunize the radar system target detection capability from received mainlobe clutter Doppler obscuring effects.

It is a further object of this invention to provide 'a velocity compensator in a moving platform. radar which is frequency-dependent.

Another object of this invention is to provide a velocity compensator in a coherent moving platform receiver that positions the centroid of the mainlobe clutter Doppler" spectrum at a frequency outside the moving target develocity compensator in a moving platform radarv which operates effectively in either the presence or absence of clutter.

Briefly, the velocity compensator in accordance with the principles of the invention is operable in a moving platform radar system having a coherent receiver, and a ground velocity information source such as an air data computer, inertial guidance, stable referenceplatforms, or Doppler navigator. The coherent I-F signal in the coherent receiver contains the Doppler information due to the motion of the platform; ground velocity V information provides a basis for prediction of the expected Doppler clutter frequency along the radar line of sight. Since the antenna scans, the antenna scan driving gimbal mechanization function produces angle information relevant to the antenna azimuth pointing angle a and vertical pointing angle 6. A radar system transmitter operates at a known frequency, therefore the radar wavelength (x) is known. For accurate vector representation 'let i represent the mainlobe frequency shift, V represent the magnitude of the velocity vector and ,5 represent the angle between the antenna axis and the velocity vector, then the following equations express mainlobe Doppler frequency shift:

fds A This equation is well known in the radar art as described in Introduction to Radar Systems by M. I. Skolnik, p. 161.

For purposes of reference and clarification of the symbols and terminology in this application related to coordinate transformation systems and vectors, further explanation is given. If the radar platform velocity has the vector components V Vj, V in a right-handed rectangular coordinate system, (I, J, K) and if the antenna elevation and azimuth angles 6 and 1 are defined by rotating the antenna axis, which is initially parallel to the I axis, through an angle 6 about the J axis and then through an 

1. A DOPPLER RETURN SIGNAL COMPENSATOR SYSTEM FOR MOVING PLATFORM RADAR, HAVING A SCANNING ANTENNA COMPRISING: OPEN-LOOP PREDICTOR MEANS RESPONSIVE TO RADAR PLATFORM MOTIONS AND ANTENNA SCAN MOTIONS TO PRODUCE AN OUTPUT SIGNAL CORRESPONDING TO THE DOPPLER FREQUENCY SHIFT DUE TO THE MOTIONS OF SAID PLATFORM AND SAID SCANNING ANTENNA; AND CLOSED-LOOP MEANS RESPONSIVE TO A CONTINUOUSLY VARYING DOPPLER FREQUENCY AND COUPLED BETWEEN SAID RADAR AND SAID OPEN-LOOP PREDICTOR MEANS TO PRODUCE AN OUTPUT SINAL CONTAINING DOPPLER COMPENSATED INFORMATION. 